Experiments have been conducted to determine whether saw cuttings are suitable for use in the manufacture of chipboards. The first industrial manufacture of chipboards using saw cuttings as a raw material began in 1948, at which time boards with a volumetric weight between 0.8 and 1.1g/cm.sup.3 were made using phenol formaldehyde resins. These boards had only limited acceptance and their manufacture soon ceased. Studies were also made of the possibility of using saw cuttings in conjunction with 8-10% of a binding agent for the manufacture of chipboard but chipboard thus produced failed to meet the high standards of industry and were moreover expensive to make.
More specifically, W. Klauditz in laboratory experiments found that the bending strength of chipboards, with a volumetric weight of 0.8 g/cm.sup.3, was about 110 kg/cm.sup.2 using unprocessed cubelike spruce saw cuttings from frame saws and about 500 kg/cm.sup.2 using 0.1-0.3 mm thick chips of 12 to 35 mm length and 5 to 7 mm width. The corresponding figures for a volumetric weight of 0.6 g/cm.sup.3 are respectively 30 kg/cm.sup.2 and 300 kg/cm.sup.2 and for a volumetric weight of 1.1 g/cm.sup.3, about 400 kg/cm.sup.2 and about 700 kg/cm.sup.2. These findings suggest that saw cuttings may be used for heavy high density chipboards but not for light low density chipboards. On the other hand, while investigations by W. Klauditz, H. J. Ulbricht and W. Kratz suggested that saw cuttings could be used for making low density chipboards, this was only possible at the sacrifice of tensile and bending strength because of the large length, width and thickness of the saw cuttings used.
In making chipboard, saw cuttings have been fractionated into different sizes. This has been done to conserve adhesive. For example, it is known that coarser fractions require less adhesive e.g. about 3-4% as against approximately 5 times this amount for finner fractions. The fine fractions excepting for their use as adhesive carriers have generally been discarded because it was believed that the chipboard properties were adversely affected thereby. On the other hand, chipboards made essentially from the coarser fractions of saw cuttings, even when optimum amounts of glue were used, consistently exhibited lower values of bending strength, tensile strength, compressive strength and bending modulus of elasticity.
To improve the properties of chipboard made from saw cuttings, about 30% of conventional chipboard materials were mixed with the saw cuttings, destined to form the surface layers of chipboards, in order to increase the bending strength and modulus of elasticity. While some improvement in these properties was obtained, it was found there was an offsetting reduction in the transverse tensile strength of the chipboard. Moreover, the conventional materials when used added to the cost of the chipboard.
O. Liiri also found that boards of saw cuttings have a lower bending strength than conventional chipboards. The transverse tensile strength while proportionately better than the bending strength was nevertheless found to be inferior to that of conventional chipboard. Liiri concluded that the bending strength was adversely affected by the fine grain cuttings from circular saws which he therefore considered to be inferior, as a raw material source, to the coarse cuttings from a frame saw. In his experiments Liiri therefore separated for use the larger frame saw derived particles from the finer circular saw derived sawdust particles; i.e. those particles which passed through a 50 mesh ASTM screen (0.30 mm openings). Like Klauditz and others, Liiri believed that improved strength of boards made of coarse saw cuttings, in particular its bending strength, would follow if the surface layer material were mixed with chip material, other than sawdust, normally used for making conventional chipboard. Even with the incorporation of such conventional chip material a fully satisfactory bending strength was not obtained. Hence, chipboards produced in the laboratory have poorer characteristics than conventionally made chipboards, particularly bending strength.
In addition to the foregoing investigations, there are a number of publications which have reported on laboratory investigations in which saw cuttings were used as a raw material for making chipboards. In the process of making the chipboard, the oversized coarse and fine fractions were, however, removed from the raw material saw cutting source in the belief that these fractions adversely affected the properties of the final chipboard.
It will be evident from the foregoing, that aside from experimental laboratory activity, there has been no commercial process until the present invention for producing chipboards exclusively of sawdust using fine sawdust cuttings from a circular saw in conjunction with the coarser sawdust cuttings from a frame saw.